Thermosetting liquid silicone rubber compositions are used in a variety of fields because they are readily moldable and after molding, have improved heat resistance and electrical insulating properties. Recently they are widely used in fixing rolls in plain paper copiers (PPC), laser beam printers and facsimile machines because of their heat resistance and parting properties.
Apparatus utilizing the electrophotographic process require means for transferring a toner image from a photoconductor drum to a paper sheet and fixing the toner image to the sheet. The most commonly used means for fixing the toner image is by passing a toner image-bearing paper sheet between a heating roll and a pressure roll rotating in close contact, thereby thermally fusing the toner image to the sheet for fixation. In this heat fusing process, the response of a copier or printer is generally accelerated by increasing the heat conductivity of the roll material. The roll materials having a high heat conductivity, however, naturally show rapid heat release. The current drive toward size reduction and cost reduction arises a need for a material having a low heat conductivity and hence, good heat insulation.
One typical such material is silicone rubber foam utilizing the low heat conductivity of gas. The silicone rubber foam is typically prepared by adding pyrolytic blowing agents. Alternatively, a silicone rubber composition is molded and cured in such a manner that hydrogen gas may evolve upon curing as a by-product whereby a foam is obtained. However, the addition of pyrolytic blowing agents has the problem that decomposition gases are toxic and odorous. Where a platinum catalyst is used as the curing catalyst, undesirably the blowing agent acts to retard curing. The method of utilizing hydrogen gas evolving upon curing suffers from the problems that hydrogen gas is explosive and the uncured composition requires careful handling during storage. Where silicone rubber foam is formed by injection molding a silicone rubber composition in a mold whereby the rubber is expanded in the mold, it is difficult to produce a silicone rubber foam having uniform micro-cells.